In Colorado, [glaciers] formed along the crests of the Front Range, the Sawatch Range, the Elk Mountains and West Elk Mountains, the Sangre de Cristo and Mosquito Ranges, the San Juan Mountains, and the Park and Gore Ranges. [Glaciation] in Colorado was selective: in many places elevation was sufficient for glaciation, but snowfall apparently was not great enough. Where they did occur, the glaciers extended down to elevations of about 8,000 feet. There, temperatures became mild enough to melt the ice.
The mountain [glaciers] have left many tell-tale signs of their presence. Valleys above 8,000 feet are U-shaped, their upper ends bounded by horseshoe-shaped, steep-walled [cirques]. In the lower portions of the valleys, at elevations just above 8,000 feet, lie long lines of glacial debris known as [moraines]: terminal moraines forming crescents across the valleys to show where melting glaciers dropped their rocky loads; lateral moraines along the sides of valleys; medial moraines where glaciers from two valleys met. Terminal moraines, often forming effective barriers across the present streams, may act as dams, creating lakes such as Grand Lake in Rocky Mountain National Park.
There were at least three distinct glacial episodes in Colorado. This is known because careful studies of glacial debris in [moraines] reveal three different degrees of rock weathering. All three stages can be seen in or near Rocky Mountain National Park. The oldest is represented by a moraine about three miles west of Estes Park, where the Big Thompson River traverses a wide U-shaped valley before entering its narrow, unglaciated canyon. The next oldest is represented in terminal moraines further up the valley, at Aspenglen campground. The youngest is shown in a prominent terminal moraine about one mile west of the park entrance in Horseshoe Park.
A large lateral [moraine] separates Hidden Valley from the south side of Horseshoe Park, and an almost equally large lateral moraine is present on the north side of this valley. At Moraine Park, both sides of the valley are edged with lateral moraines also.
Studies in Rocky Mountain National Park have revealed many other details of [glaciation] in this area. These are described in Park Service brochures and guidebooks, in the museum at Park headquarters, and in informative roadside signs.
A line of hikers approaches Arapaho [Glacier], west of Boulder. Movement of the glacier is evidenced by the crevasses apparent just below the snowfield in the dirty gray glacial ice. (H. H. Heuston photo)
Several small [glaciers] are still present in the Colorado mountains, all in sheltered [cirques] above 11,000 feet. These may be remnants of the former larger glaciers, or new glaciers formed after a long warming episode. A hike to one of these glaciers is a rewarding experience for anyone interested in geology. Some of the more accessible are St. Mary’s Glacier west of Denver, Arapaho Glacier west of Boulder (the Boulder Chamber of Commerce sponsors a festive hike to Arapaho Glacier every August), and Tyndall Glacier in Rocky Mountain National Park.
The Ice Age brought drastic changes also to the landscape below 8,000 feet elevation. Heavily loaded with glacial debris, mountain streams disgorged coarse sands and gravels along the mountain front and in the intermontane basins. As the [glaciers] melted after each period of expansion, the swollen streams cut deeply into their former deposits and into much older rocks as well. Royal Gorge, the Black Canyon of the Gunnison, and many of the deep, colorful canyons of the [Plateau] Province were cut or at least deepened by these waters. The canyons along the east face of the mountains—Big Thompson, Boulder, Clear Creek, and others—were also deepened and sharpened by the rushing ice-fed torrents.
On the prairies, rivers dumping their loads of sand covered the older rocks. Sand dunes developed along the river channels. Bones and huge tusks of hairy mammoths were sometimes buried in these soft deposits; now they are occasionally revealed as the dune and river sands are washed or blown away by continuing erosion.